The invention relates to making an ultra-refractory material that withstands high temperatures in an oxidizing medium, in particular in the presence of air, of water vapor, and more generally in the presence of any gaseous or liquid phase containing oxygen or an oxygen compound.
The invention relates in particular to making refractory material parts that are suitable for constituting protection that withstands an oxidizing medium at high temperature.
The invention also relates to protecting thermostructural composite materials made up of fiber reinforcement densified by a matrix against high temperatures in an oxidizing medium. More particularly, but not exclusively, the invention relates to thermostructural composite materials containing carbon and/or silicon carbide (SiC), such as carbon/carbon (C/C) composite materials that are constituted by carbon fiber reinforcement densified by a carbon matrix, and to ceramic matrix composite materials in which the fibers and/or the matrix contain SiC.
The invention also relates to protecting monolithic materials based on carbon (e.g. graphite) or on SiC-based ceramic against high temperatures.
Thermostructural composite materials are characterized by their mechanical properties that make them suitable for constituting structural parts, and by their ability to conserve those mechanical properties at high temperatures. Nevertheless, when they contain carbon, composite materials present the major drawback of oxidizing as from 400° C. in air or in an oxidizing medium and of losing their thermostructural properties in part.
Furthermore, with monolithic ceramic materials or composite materials comprising SiC, the SiC oxidizes in two modes. The first mode corresponds to so-called “passive” oxidation, which occurs under a high partial pressure of oxygen and at a temperature that is relatively low, the SiC then becoming covered in a layer of silica. The second mode, known as “active oxidation”, occurs when the SiC is raised to very high temperature under a low partial pressure of oxygen, the SiC then being consumed rapidly since all of the oxides that are formed are in the gaseous state.
With C/C composite materials, it is known to use protective layers made up of ultra-refractory single-layer deposits based on hafnium diboride (HfB2) or of zirconium diboride (ZrB2). Among the various systems fabricated by mixing (Zr/Hf)B2 and SiC, one of the most widely-used is that comprising 20% by volume SiC (giving an atomic ratio (Zr or Hf)/Si=2.7), possibly with additives (RE2O3 preferably up to 3% by volume (where RE designates a rare earth comprising yttrium (Y) and the lanthanides), or REB6, MoSi2, or AN preferably at 10% by volume), or a mixture of those compositions.
Nevertheless, that type of protection material presents two drawbacks, namely:
a coefficient of thermal expansion that is too great relative to that to the C/C material, thereby leading to cracks appearing in the ultra-refractory layer and to loss of cohesion along the interface between the C/C material and the layer. The cracks created in this way then become paths for diffusing oxygen and water (if any is present in the environment in use), thereby leading to the C/C substrate being oxidized and having its mechanical properties weakened or even lost; and
poor resistance to oxidation at temperatures higher than 2300° C.
In order to mitigate the first above-mentioned drawback, an underlayer based on SiC alone has been introduced between the C/C substrate and the ultra-refractory layer so as to provide a layer for matching coefficients of thermal expansion. Nevertheless, that solution is not considered as being sufficiently satisfactory since, depending on utilization conditions, SiC oxidizes either passively by becoming covered in a layer of silica which interacts with the ultra-refractory layer, or else actively, which leads to pores being formed in the SiC layer, or even to loss of cohesion.
The documents “High temperature oxidation-resistant hafnium-tantalum alloys” by K. Marnoch, J. Metais 1225 (1965) and “Oxidation of refractory metallic coatings on carbon fibers heated up to 1850° C.” by A.-S. Andréani et al., ICMCTF No. 37, San Diego, 2010, Vol. 205, No. 5 (482 p.) pp. 1262-1267, propose using alloys of hafnium (Hf) and of tantalum (Ta) or HfC—Ta, Hf—TaB2, or Hf—TaC mixed compositions in order to improve the oxidation resistance of ultra-refractory systems. Although those systems give results that are satisfactory in air, they cannot be used in the presence of water or water vapor because of the great instability of the metals Hf and Ta, since these metals generate explosive vapors in the presence of water.
Although such compositions do indeed resist oxidation in air at temperatures higher than 2000° C., they cannot be used in the presence of water because the metallic materials Hf and Ta are unstable in the presence of water. That drawback limits the field of utilization of such compositions by excluding applications that involve atmospheres containing water. Furthermore, certain techniques for preparing materials, such as for example a liquid technique in which an aqueous solvent might be used, likewise cannot be used. Finally, they give rise to a problem of storing them in powder form since it is necessary to guarantee that they are stored with no moisture.
There thus exists a need for a protection material that resists oxidation at temperatures higher than 2000° C., and in particular in the presence of a wet environment (water present).
This applies in particular to components for rocket engines or for aeroengines of the turbojet type in which the water vapor and the carbon dioxide that are produced and ejected through the nozzle create an environment that is wet and oxidizing. This protection problem also occurs for vehicle heat shields for re-entry into the atmosphere.